1. Field of the Invention
This invention relates to a high rate electroplating cell suitable for electroplating alloys through patterned masks. In particular the cell is suitable for high speed plating of highly uniform Ni-Fe (permalloy) magnetic layers, through patterned masks, in the manufacturing of Thin Film Head (TFH) or Magnetic Bubble devices.
2. Background of the Invention
Precision electroplating often requires high degree of uniformities. These include thickness uniformity and, in the case of alloy plating, composition uniformity. Uniformities are further defined as macro-uniformity (over relatively large dimensions of about 1 cm, or larger, such as across a wafer), and micro-uniformity (over small dimensions of a few millimeters, or smaller, such as across an individual micro-device or a die). When plating an alloy through a patterned mask, such as a photoresist mask, composition non-uniformity is often encountered among opening areas of different aspect ratios. Such micro-non-uniformity is due to insufficient agitation and replenishment of the minor constituent(s) inside deep and narrow opening areas. An example of such a situation is the plating of Ni-Fe (permalloy) through a patterned photoresist mask in the course of manufacturing Thin Film Heads (TFH) or Magnetic Bubbles. In particular, plating the top pole layer in advanced TFH devices presents demanding challenges due to severe variations of the topography and aspect ratio across a device. While the narrow pole-tip (about 5-7 .mu.m wide) is located on a flat surface, the wide (about 50-75 .mu.m) back-yoke is located over an elevated step (comprising coil and insulation layers), about 10-15 .mu.m above the pole-tip. The photoresist mask is only about 4-5 .mu.m thick in the back-yoke area, but about 12-17 .mu.m thick in the pole-tip area. Thus the aspect ratio, defined as the ratio between the vertical dimension (or thickness of the photoresist mask) to the lateral dimension of an opening, varies across a device from about 3:1 or greater in the pole-tip area to about 1:10 or less in the back-yoke area. This large variation in the aspect ratio across a device gives rise to severe composition micro non-uniformity. Fe.sup.+2 ion concentration in the electrolyte is very low compared with the Ni.sup.+2 ion concentration. The ratio between the two is typically only about 0.015-0.030. In comparison, the composition ratio between Fe and Ni in the deposit permalloy is about 0.20-0.25. As a result, stagnation and Fe.sup.+2 ion depletion occurs to further extent in openings of larger aspect ratio than in openings of smaller aspect ratio. This leads to depletion of iron content in the plated Ni-Fe alloy at the pole-tip area, compared with the back-yoke area. Ni-Fe composition uniformity is critical for adequate TFH device performance. U.S. Pat. No. 3,652,442 to Powers et al. discloses a paddle cell designed to improve the uniformities of plated Ni-Fe in TFH devices. That patent advocates non-turbulent laminar flow of the electrolyte in the vicinity of the cathode (consisting of a wafer substrate) surface. However, with increasing aspect ratios of patterned features, non-turbulent laminar flow parallel to the cathode (or substrate) surface becomes ineffective for supplying fresh solution and replenishing the minor component(s) inside deep and narrow feature openings (having high aspect ratios). At the same time, wider feature openings with lower aspect ratios receive better supply and replenishment, resulting in poor composition micro-uniformity. In order to improve the composition micro-uniformity, the plating current density (and rate) must be reduced. The effect of current density on the various types of uniformities and the necessity to decrease it in order to improve the uniformities was disclosed in U.S. Pat. No. 4,102,756 to Castellani et al. and in U.S. Pat. No. 4,279,707 to Anderson et al. However, lower current density, or plating rate, results in lower throughput, thus adversely affecting the process economy.
In general, the uniformities degrade with increasing substrate dimensions and with decreasing feature size. These are precisely the current trends in the manufacturing of TFH and Magnetic Bubble devices. Larger wafers and smaller devices increase the number of devices per wafer, thereby reducing the processing cost per device. Smaller features are required to increase the recording density. Also, macro-non-uniformity of the current distribution across a wafer (such as due to radial distribution or edge or corner effects) leads to both thickness and composition macro-non-uniformities. A rotary (wafer or cathode) cell was disclosed by Grandia et al. in U.S. Pat. No. 4,304,641. That patent advocates nozzles of increasing size and uniformly spaced, or the same sized nozzles with decreasing radial spacing, in order to provide a differential radial flow distribution on the wafer-cathode. It provides increasing flow rate along the wafer's radius in order to improve thickness macro-uniformity. The technique relies on decreasing current efficiency with increasing flow rate, as described by Andricacos et al. in Journal Of Electrochemical Society, Vol. 136, No. 6, pp. 1336-1340 (1989). However, in addition to decreasing current efficiency, increase of the flow rate also results in sharp increase of the iron content in deposited permalloy film, as described by Andricacos et al. Uniformity of the permalloy composition is most critical for proper performance of the TFH device. The techniques disclosed in the Grandia patent were mainly applied in the fabrication of magnetic bubbles where the topography is relatively flat and the plated film thickness is less than 0.5 .mu.m, thus requiring low aspect ratios. The technique may not provide sufficient agitation inside features with high aspect ratios such as in TFH devices and, therefore, does not improve micro-uniformities. The problem is particularly acute in areas near the center of the wafer, which receive reduced flow. The cell of the Grandia patent requires an even lower plating rate (about 0.05 .mu.m/min) than the paddle cell of the Powers patent (about 0.09 .mu.m/min) in order to maintain acceptable micro-uniformities. It does not offer an advantage, in this respect, over the paddle cell of the Powers patent.